The present invention concerns a process for coating of paper, board and similar cellulosic materials.
According to such a process, an aqueous coating colour is applied to the surface of a web.
1. Field of the Invention
The present invention also concerns a coating colour and the use thereof for coating of paper and board.
Papers and boards are provided with various mineral coatings in order to improve the printability of the products, i.e., to improve the properties of the printing surface and the printing process. The objective of the coating is to cover the fibres and fibrous flocks of the paper or board and thereby decrease the roughness of the surface and the size of the surface pores. The coatings usually consist of pigments and binding agents and various additives.
2. Description of Related Art
It is known in the art to perform coating by applying, in relation to the final coating amount, a manifold amount of coating colour, which is then scraped to the final amount, usually with a blade. The aim is to obtain the best possible coverage and other desired properties with a minimal use of coating colour. A further objective is to perform coating at high speed. One of the problems related to the doctor blade coating technique described above is that a large amount of the mixture has to be recycled.
One of the alternative, new coating methods, is a method known as film press. It is based on transferring the coating colour onto the material to be coated in a nip consisting of two rolls. The coating colour is applied on the roll and some of the applied amount is transferred to the web. The amount that is transferred depends on the properties of the mixture and the base web. The amount that is recycled into circulation is much smaller in the film press method than in other usual coating methods and therefore also smaller changes occur in the mixture with longer application times. The advantages of the film press method include the possibility to obtain a large coat weight area at higher speed than before, and the fact that the coverage of the base web, even with small amounts of coating, is better than with the doctor blade method.
Draining is considered to be the mechanism leading to coating and the formation of the coating structure. Because the pores of the base web, which acts as a filter, are larger than the average particle size of the mixture, it is essential to prevent the infiltration of the mixture components into the web pores, in order to obtain a high coverage. The faster the mixture attains its immobilisation (solidification) point--a state, in which the particles no longer can move in relation to each other--the smaller is the amount of the mixture that infiltrates the pores of the paper. Thereby the mixture covers better the web to be coated. The coverage of the mixture is an essential factor when smaller amounts of coating are desired.
Mist-formation, i.e., the formation of drops of the coating colour when the nip opens, is a problem related to operating the method of film press at high speed. The emitted mixture particles can land on the coated web and also contaminate the coater and its environment. When transferring the mixture applied to the roll in the nip onto the surface of the web to be coated, part of this mixture layer solidifies to a state, in which it no longer splits when the nip is opened. Part of the mixture layer remains unsolidified.
According to recent knowledge, mist-formation depends primarily on the thickness of the unsolidified splitting layer of the mixture and the splitting speed of the film (which depends on, i.a., the operating speed and the diameter of the rolls). When the coating layer sets quickly and the solidified layer is thick, the thickness of the free layer, which is emitted as a result of splitting, remains small. In these circumstances, mist-formation is minimized. Also the amount of coating in the film press method is determined by the total sum of the solidified layer and the mixture layer remaining on the base web in the splitting phase of the unsolidified mixture layer. The fraction of the mixture applied on the roll that is transferred onto the web to be coated is larger; and the amount of mixture recycled is smaller, when using mixtures that solidify quickly and have a low immobilisation point, than when using mixtures that solidify slowly and have a high immobilisation point.
Interaction between the components of the coating will influence the solidification of the coating. The following means are available for speeding up immobilisation of the coating and, thus, for improving the coverage of the coating and, in coating processes based on film press, for decreasing the amount of mist-formation and reducing the amount of the coating in circulation:
1) Removal of water from the mixture by the use of an absorbing base paper and by using a mixture which has poor water retention; rapid removal of water causes rapid solidification of the coating colour; PA0 2) Use of a coating colour which has a high dry matter content at coating so as strongly to increase the viscosity in the nip already at a small increase of the dry matter content; PA0 3) Use of cationic components in an anionic coating mixture.
In the first case, the properties of the circulating mixture are changed constantly during the process (e.g. the dry matter content increases and the binder concentration decreases). At high coating speeds in the film press method, no improvements can be obtained with this method.
The second alternative does not influence the transferred amount of mixture at high coating speeds (over 1200 m/min) in the film press method, but mist-formation is reduced when the dry matter content increases. The increase of the dry matter content of the mixture is limited by the dry matter contents of the components and the interactions between them (viscosity). In the third case, cationic components can be used in anionic paper making processes only to a limited extent.